The present invention is based on a piezoelectric actuator as is known, for example, from German Published Patent Application No. 37 13 697.
As schematically shown in FIG. 3, piezoelectric actuators are usually composed of several plates 2, xe2x80x9cpiezoelectric elementsxe2x80x9d, of a piezoelectric material arranged stack-like one over another, the stacking direction being selected to run in the joint polarization direction of the piezoelectric plates 2. At two side faces of the stack, i.e., the piezo stack, in each case one outer electrode 3, 4 is attached, respectively, which can be electrically connected to a contact 12a, 12b of a control voltage source 11. Internal electrodes 7, 9 of piezoelectric actuator 1 are arranged between the piezoelectric plates 2, respectively. Internal electrodes 7, 9 are electrically plated through alternately to only one of the outer electrodes 3, 4, respectively, so that neighboring internal electrodes 7 and 9 arranged one over another are electrically connected to the different contacts 12a, 12b of control voltage source 11, respectively.
In this arrangement of a piezoelectric actuator, each piezoelectric element 2 is joined to an electrode 7, 9 on both plate surfaces, it being possible to apply a voltage to the electrodes via outer electrodes 3, 4. When the voltage is applied, each of the plate-like piezoelectric elements 2 arranged stack-like one over another expands in the direction of the electrical field developing between electrodes 7, 9, whose direction coincides with the polarization direction of piezoelectric plates 2. Due to the great number of stacked piezoelectric elements 2, it is possible to achieve a relatively long stroke of the overall arrangement, using a relatively low control voltage at the same time.
Piezoelectric actuators of the type above described can be used for various purposes such as for actuating a valve-closure member of a fuel injector, for actuating hydraulic valves, for driving micropumps, for actuating electrical relays and the like. Various such uses are already known from the related art.
European Patent No. 0 361 480, for example, describes a fuel injection nozzle for combustion engines in which the valve needle is set in lifting movements for opening and closing the injection nozzle. In this context, the driving element for these lifting movements is composed of a stack of plates capable of being piezoelectrically excited which are provided with flat electrodes.
A further injector for fuel-injection systems in internal combustion engines such as direct-injection diesel engines and the like, is known from German Published Patent Application No. 35 33 085, which has also a piezoelectrical final control element for opening and closing the valve by a lifting movement, i.e., displacing the valve needle. Here, the piezoelectrical final control element is also composed of a plurality of plate-shaped piezoelectric elements and can be axially expanded or contracted by an applied voltage in a very short time.
Furthermore, German Published Patent Application No. 38 00 203 describes a fuel injector having a piezoceramic valve control element composed of piezoelectric ceramic plates stacked one over another, having voltage feed lines to each piezoelectric ceramic plate. The special feature of the piezoelectric actuator used for this injector is that paired ceramic plates of opposite polarization are stacked one over another for enlarging the travel of the piezoceramic valve in this way.
A further use of piezoelectric actuators described in European Published Patent Application No. 0 477 400 relates to an arrangement for a travel transformer of a piezoelectric actuator for enlarging the stroke of the piezoelectric actuator.
In the above documents cited by way of example, each of which discloses a possible use of piezoelectric actuators of the type specified at the outset, the design and the method of functioning of the used piezoelectric actuators are, in fact, not described in detail, however, they essentially correspond to those of the actuators above described on the basis of FIG. 3.
In the case of conventional piezoelectric actuators, because of the connection of the stacked piezoelectric elements 2 and internal electrodes 7, 9 to the two outer electrodes 3, 4, the piezoelectrically generated expansion mainly occurs only in the central area where internal electrodes 7 and 9 are opposed. In edge zones 13, where internal electrodes 7 and 9 are not directly opposed, an area exhibiting changed field strength, and, consequently, also tensile stress forms. Frequently, cracks form in such actuators because of this tensile stress. The formation of such cracks is further explained in the following on the basis of FIGS. 4A and B. In this context, FIG. 4A shows actuator 1 in the neutral state, i.e., without applied control voltage, and FIG. 4B shows actuator 1 in operation, i.e., with applied control voltage and resulting expansion of piezoelectric elements 2.
FIGS. 4A and 4B show section IV of a conventional piezoelectric actuator according to FIG. 3 in an enlarged view. In the boundary region between the stack of piezoelectric elements 2 and internal electrodes 7, 9 and outer electrodes 3, 4, two areas 13, 14 can be distinguished. In the one area 13, internal electrode 7 is not plated through to outer electrode 4, and the ceramic normally used for piezoelectric element 2 is sintered through in this area 13. In the other area 14, internal electrode 9 is led through to outer electrode 4 but does not contact the other outer electrode 3. The adhesive strength in the second area 14, i.e., between piezoelectric element 2 and internal electrode 9 is by a factor of 3 to 5 less than the adhesive strength in area 13 within the piezoelectric material. As shown in FIG. 4B, applying a control voltage produces expansions which are greater in the middle of actuator 1 than in the boundary region to the outer electrodes. Due to the resulting high tensile stress in areas 13, 14 of actuator 1, cracks 15 form frequently along the boundary between internal electrodes 9 and piezoelectric elements 2 in area 14. During the continued operation of actuator 1, these cracks 15 extend into outer electrode 4, whereby the contacting of internal electrodes 9 is, at least partially, considerably deteriorated or even interrupted, thus reducing the overall expansion of actuator 1.
The piezoelectric actuator according to the present invention has the advantage that, because of the actuator design having two groups of at least two outer electrodes, respectively, the areas where the internal electrodes are plated through to the outer electrodes are distributed over more side faces of the actuator, consequently spaced further from each other in the stacking direction, and, at the same time, it being possible to sinter through three edge zones at each internal electrode. By this measure, the tensile stress in the actuator can be reduced, thus markedly diminishing the cracking tendency in general and, in particular, in the edge zones. A further advantage of the present invention is that the heat dissipation of the actuator is markedly improved by using a total of four outer electrodes.
The cyclic contacting sequence between the internal electrodes and the different outer electrodes is particularly advantageous. As a result, the throughplating areas are uniformly distributed over the entire actuator and maximally spaced from each other at the same time, thus considerably facilitating the possibilities of bridging cracks in the outer electrodes in the event of a possible formation of cracks. According to a refinement of the present invention, undulated electrodes are mounted to the outer surfaces of the outer electrodes for such a bridging of cracks, the wavelength of the undulated electrodes being four times the distance between two consecutive internal electrodes. In conventional actuators, the wavelength would only be half as long, and there would be a risk that the undulated electrodes could be soldered to the outer electrodes in a flat-spread manner.